Cysteine residues in the C-terminus of the neutral- and basic-amino-acid transporter heavy-chain subunit contribute to functional properties of the system b0,+-type amino acid transporter

The neutral- and basic-amino-acid-transport glycoprotein NBAT (rBAT, D2) expressed in renal and jejunal brush-border membranes interacts with the b^0,+AT permease to produce a heteromeric transporter effecting amino acid and cystine absorption. NBAT mutations result in type I cystinuria. The b^0,+AT permease is presumed to be the catalytic subunit, but we have been investigating the possibility that cysteine residues within the C-terminus of NBAT are also important for expression of transport function. NBAT mutants were produced with combinations of Cys^664/671/683 → Ala substitutions. Mutants with Cys⁶⁶⁴ → Ala show decreased arginine and cystine transport and specifically lose sensitivity to inhibition of transport by the thiol-group reagent N-ethylmaleimide (NEM). We suggest that the C-terminus of NBAT may have a direct role in the mechanism of System b^0,+ transport (the major transport activity defective in type I cystinuria) and that Cys⁶⁶⁴ of NBAT is the major target for NEM-induced inactivation of the transport mechanism.